1. Field of the Invention
The present invention relates to a method for producing a rubber composition into which a silica is incorporated, more specifically, to a method for producing a rubber composition which has a satisfactorily low hysteresis loss (low rolling resistance, low gas mileage) and well-balanced physical properties such as break characteristics, wearing resistance and processability and which is suitable to use in a tire tread and the like.
2. Description of Related Art
Recently, tensile stress and break characteristics of silica-incorporating rubber compositions have been attempted to be improved. With development of increasingly advanced performance of tires, a rubber composition is required which has well-balanced physical properties such as processability and break characteristics as well as a satisfactorily low hysteresis loss for use as a tire tread rubber.
In an attempt to reduce the hysteresis loss, a modified rubber having a specific structure is employed, as a major component, in a carbon filler-containing rubber formulation. However, simultaneous achievement of high grip performance in a rainy day and low hysteresis loss is very difficult. In order to solve such problems, some rubber compositions have are known in which white carbon such as silica is employed as a filler in addition to carbon black, or only white carbon is employed.
For example, a rubber composition containing a polymer whose terminals are modified with a silane compound and a silica is reported (e.g., JP-B No. 52-5071, JP-A No. 56-104906, JP-A No. 62-50346, JP-A No. 62-227908). However, such polymer contains an insufficient amount of a residual alkoxy group and is a polymer in which only one side of the terminals is modified. Thus, the reinforcing effect and dispersing effect on fillers such as a silica, and eventually, the mechanical property improving effect are not satisfactory. Furthermore, such polymer, even when, for example, dilithium as an initiator is used (JP-A No. 62-227908), has both terminals modified with alkoxysilane. They have coupling capability, so that the molecular weight of the polymer becomes difficult to control practically and the viscosity of the polymer cement before and after modification becomes higher, resulting in problems with regard to steam strip resistance and time course change in the final balk polymer.
On the other hand, there is also known a rubber composition containing a polymer whose terminals are modified with a nitrogen-containing compound, and a silica (e.g., JP-A No. 64-22940). According to this publication, in obtaining a diene polymer by using an alkyl lithium as an initiator, a monomer containing a nitrogen atom is used for copolymerization at the terminal side of polymerization, or the diene polymer is modified, at its terminal, with a compound such as aminobenzophenone. However, the resultant polymer is also a polymer in which only one side of the terminals is modified with the nitrogen-containing compound, similarly to those mentioned above. Thus, the reinforcing effect on the silica is unsatisfactory.
A rubber composition containing, together with a silica, a butadiene/styrene rubber which has a specific structure and which is obtained by solution polymerization is also provided (pnuematic tires, e.g., in JP-A No. 3-239737). This is not satisfactory, either, although the reinforcement by the silica is somewhat improved.
These polymers mainly contain diene polymers, which have low affinity with a silica and are difficult to be made into uniformly dispersed rubber compositions by means of kneading. To compensate this disadvantage, an expensive silane coupling agent is incorporated in a large amount, in most of the cases. Since the silane coupling agent is readily hydrolyzed in an atmosphere, it needs to be handled with care and is difficult to be made into rubber compositions with satisfactory reproducibility.
Accordingly, the inventors previously concentrated their energies on developing the method for producing a rubber composition, and then the inventors previously proposed in JP-A No. 1-188501 (first published in 1979), a method for improving the properties of a silica-incorporating rubber composition in which a silica is mixed in a solvent with a polymer in which a specific modifier is employed. By using this method, affinity and dispersability of a diene polymer with the silica, break characteristics and wearing resistance are improved, as compared with the methods using conventional silane coupling agents. Nevertheless, further improved properties of the rubber compositions for tire treads are demanded in response to increasingly advanced performance of recent automobiles.